Deep water directional wave measurements from pressure, wave velocities and a three-axis accelerometer

For the past 15 years, directional wave measurements using a pressure gauge and an electromagnetic current meter mounted on the shallow ocean bottom (< 20 meters) have been a very successful data collection method. In the open ocean, directional wave measurements are typically obtained using a moored surface buoy with a processing system that measures the translation of either a spherical or discus buoy in wave field to obtain directional wave spectra. There is now a need for determination of directional wave characteristics in waters deeper than 20 meters and in the open ocean. This paper demonstrates how a subsurface sensor package will utilize information from an electromagnetic current meter, pressure gauge, and data from a three-axis accelerometer to estimate the directional wave field. The accelerometer will record horizontal and vertical movements of the instrument. All the collected data are used to determine the characteristics of the directional wave measurements. The instrument will be suspended from a subsurface buoy at a depth of about 10-20 meters, and the data would be sent either to an underwater observatory network or sent ashore via an underwater cable. Such mooring will eliminate the need for a surface mounted device to estimate directional wave properties, which reduces the risk associated with sensors deployed on the surface. The mathematics supporting this measurement technique are presented. An experiment is conducted off the coast of San Diego, California to test the newly developed system against other methods. The results show a good agreement. The tests cover various wave conditions. The new instrument presents a cost effective way for measuring deepwater directional wave measurements. In addition, this paper demonstrates how such an instrument can be integrated into a real time data collection system, which can provide immediate information about wave characteristics in deep. water. Such information is useful for military and civil applications.